1. Field of the Invention
This invention relates generally to solid-state devices and more particularly to a solid-state signal amplifier, the use of which contemplates an inductive load in the output circuit of the amplifier.
2. Discussion of the Prior Art
In recent years steps have been undertaken in the communication industry and particularly the telephone industry to replace vacuum tubes that are in existing circuits with solid-state devices. Such solid-state replacement devices advantageously use one or more n-channel junction field-effect transistors as active elements. N-channel junction field-effect transistors are well suited for one-for-one vacuum tube replacement applications because their current flow from a source of negative or n-type conductivity through an n-type channel to a drain of n-type conductivity is controlled by a gate of p-type conductivity which forms a p-n junction with the channel. Similarly, to grid voltages on vacuum tubes, gate voltages applied to the n-channel field-effect transistors are normally slightly negative with respect to ground, and they increase the drain current as they become more positive. In replacing vacuum tubes with n-channel field-effect transistor circuits, the transconductance (gm) of the transistor circuits is chosen to be equivalent to that of the replaced vacuum tubes. Consequently, for a given change in the voltage applied to either the grid or gate, a predetermined change in the plate or drain current results.
The field-effect transistors have advantages over vacuum tubes with similar characteristics in that they consume less power than the vacuum tubes they replace. In addition, such solid state replacements for the vacuum tubes appear to have a longer life span and more stable characteristics over the active service life than their vacuum tube counterparts.
Attempts to replace a twin triode vacuum tube in a channel regulator of a telecommunication carrier system have met with considerable difficulty. In this carrier system, voice frequency communication signals are received as amplitude modulations of a basic carrier frequency signal. The carrier frequency signal is applied to the channel regulator unit through an input transformer. A signal output from the transformer is applied to a two-stage amplifier circuit. The output from the amplifier circuit is coupled through an output transformer to a demodulator circuit which rectifies the carrier frequency. The negative potential of the rectified or demodulated carrier frequency is fed back and algebraically added to a bias voltage to control the amplification of the signal. Thus, a decline in the carrier signal amplitude results in a less negative feedback and in a consequently increased signal amplification. Conversely, an increase in the carrier signal results in a more negative feedback and net bias of the amplifier and a lesser amplification of the carrier signal.
The gain of the amplifier in the channel regulator unit is substantially inversely proportional to the input amplitude or level of the carrier frequency. The voice frequency, being an amplitude variation of the carrier frequency, is amplified proportionately to the amplification of the carrier frequency in the channel regulator unit. The channel regulator unit consequently automatically controls the output amplitude of the voice frequency signal.
In replacing the twin triode amplifier tube of the prior art unit with a solid state amplifier device, such an automatic gain control of the applied signal is lost if the net negative DC feedback from the demodulated carrier signal does not correspond to a voltage feedback required to control the gain of the amplifier device. In prior attempts to replace each of the stages of the prior art twin triode tube with junction field-effect transistor circuits a lack of regulation of the signals, an inability to obtain desired output signal levels at some carrier frequencies, and various other problems resulted.
From the foregoing, it appears desirable to have a circuit with the advantages of a solid-state circuit and the characteristics and properties of a twin triode tube, the gain of which can be controlled by the feedback of a negative biased voltage which results from the demodulation of the output of the amplifier device.